This invention is in the field of wireless communication and Media Access Control (MAC) Protocols with extensions and methods designed to improve performance in the communication environment.
In a typical wireless communication network environment that has a number of stations, each device (network station) can be considered as having a MAC layer and a Link layer, and would require the use of MAC and Link layer protocols. The Link layer is responsible for providing address discovery, address conflict resolution, connection setup, information data exchange and disconnection services. In some networks high latency is often a big consideration due to the setup overhead required to gain access to the media. In a typical networking environment the Link layer protocol will be responsible for ensuring end to end delivery of the data. To accomplish this a typical link layer will incorporate a polling process by which control frames are periodically exchanged between the source and target device to acknowledge reception of the sent data frames. In networks that suffer from high latency the overhead incurred by utilizing this type of polling process can be very expensive (measured in terms of throughput). The cumulative effect for the entire network when considering the aggregate cost of all devices in the network is even more expensive. In a wireless network latency can be measured by the time it takes for a station to get access to the channel.
In a wireless environment, the bit-error-rate depends on the received signal quality at any specific station and on the signal-to-noise ratio (SNR) at the receiver. In general, the SNR depends on the distance of the receiver from the transmitter, the transmitted power, and the environment itself (e.g. open space, characteristics of the geographical space and materials used in the environment). Assuming a fixed transmission power and a given environment, the SNR at any receiver depends on the distance from the transmitter as well as the level of interference (e.g. measured in terms of power) at the receiver. This interference can be generated by the environment (e.g. light sources of Infrared transmission) or by the signal transmitted by other terminals. In general, depending on the interference characteristics, modulation, coding, or signal processing techniques can be used to improve the SNR at a receiver.[1] For the Infrared wireless medium, a process based on repetition coding has been proposed in [2] In this process, each symbol is transmitted n times (hence the term repetition coding) in the wireless channel. We refer to n as the repetition rate (R). The receiver in turn receives n symbols and makes a decoding decision. Now, as we increase n, the probability of receiving a symbol correctly increases and for a given bit-error-rate (BER) or signal-to-noise-ratio (SNR) at any receiver, one can find n in such a way that the probability of receiving a correct symbol is above a predefined level. As a result, the repetition rate R required to receive a symbol correctly with a predefined probability at a given receiver depends on the interference level at the receiver as well as its distance from the transmitter. Hence, since the SNR depends on the geographical placement and interference, the repetition rate necessary to achieve a given BER at a receiver is not fixed for all connections within a wireless network.
Distributed MAC protocols in a wireless CSMA/CA network are often plagued by asymmetric or hidden nodes. One common process to deal with this problem is an RTS/CTS style of MAC protocol. Now let us consider the problem of accessing the shared medium using any distributed or coordinated protocol.[2] In general, if any wireless terminal using a media access control (MAC) protocol of choice needs to transmit signals for coordinating the access to the medium, such signals need to be heard by all terminals using that medium. We refer to any signal that bears information important to the MAC protocol, media coordination, or reservation as reservation or control symbol (the control symbol can be sent from any wireless terminal in a distributed MAC and by a central coordination in a coordinated MAC). The collection of all reservation symbols in each frame conveys the reservation information that is used to follow the MAC protocol rules and specifications. There are other type of signals or symbols which we refer to as information or data symbols which are used for transferring information such as higher layer protocol data units from a transmitter to a specific receiver or a group of receivers (in case of multicast). These symbols do not bear any reservation or control meaning and hence do not need to be heard by all terminals using a shared wireless medium. Now, if reservation or control symbols are not heard by all terminals that use the shared medium, the MAC protocol rules will not be followed correctly by all stations and any station that does not receive the reservation signal might try to access the medium without being permitted. As a result, collisions may occur with a high probability and depending on the MAC protocol, the network throughput can degrade. In other words, the reservation reliability depends on the probability that all stations accessing a shared medium receive a reservation symbol and in turn the media throughput depends on the reservation reliability. Here one important issue in the design of the MAC protocol is the choice of the repetition rate R. Let us define C(I,J) as the transmission rate from station I to station J, such that the received symbol error probability at J is less than a predefined level. For a given maximum transmission rate of Cmax with R=1 where each symbols is sent only once, Cmax/C(I,J) defines the repetition rate R(I,J) that a terminal I uses to transmit symbols to J. As for choosing the repetition rate R(I,J), one can choose to transmit all symbols at the maximum repetition coding rate such that all stations accessing the shared medium can hear all transmissions (reservation and data). But this will result in the lowest achievable throughput.
The method described herein provides a process where all symbols within control (or reservation) frames and/or Data frames are sent with a repetition rate Rmax which is high enough that all stations within the interference range can decode the symbols correctly with a high probability. Consider a random access protocol based on Request-to-Send (RTS) and Clear-to-Send (CTS) as described in.[4] In the light of the above discussion on multirate communication using repetition coding, RTS and CTS packets need to be sent with a repetition coding rate Rmax which enables all terminals that share the wireless medium to receive such control packets or frames with a predefined high probability. The problem in this case is that first of all, highly repetition coded RTS or CTS packets will increase their transmission time and hence the collision window of the MAC protocol and also reduce the throughput. In addition, when the channel is reserved for an extended time by doing a burst reservation, there is a need for other terminals that are not participating in the reservation to know that the channel is in use and to hold back any transmission. Note that even when the reservation is done with Rmax, there is a possibility that some station may miss the reservation exchange. Again, in order to solve this problem, one could send all information with Rmax, but this will result in a very low throughput.
The solution to this problem as defined in [5] is to have the repetition rate R for the body of control frames less than or equal to Rmax. That is the body of control frames are transmitted with a repetition rate so that their destination can receive and decode the body with a high probability. The header is always repetition coded with Rmax so that all stations within the interference range of a transmitter can receive and decode it with a high probability. This process is designed to increase the throughput and reduce the collision window on the transmission of reservation control frames (e.g. RTS/CTS) since if MAC bodies were transmitted with the repetition code of Rmax they will have a potentially much longer transmission time and hence a larger collision window. All header fields of any frame (reservation frames or data frames) that bear any reservation specific information are repetition coded with Rmax and are in the frame header. The fields defined below are used in the header in addition to any preamble (or any other fields) required by the physical layer. We assume that source and destination addresses are within the frame body and are sent with the repetition rate R.
1xe2x80x94The Reservation Identification (RID) field identifies an ID associated with an ongoing reservation attempt or data exchange. Since RID is in the robust header, it is heard with a high probability by all stations with which the transmitting station can interfere. The RID is defined per reservation in a static or a random manner. That is a station that starts a reservation, can have a predefined RID defined uniquely for each destination station, or it can select a random RID for the full duration of each reservation attempt and data exchange. Another alternative use of the RID is to define it for a group of stations. In this case all stations with the same RID would have a common repetition rate R which enables them all to receive and decode the transmission of any member of the group. Any station that receives a data or control frame with a RID assigned to a group different from the one (or ones) assigned to that station, would ignore the transmission. In other words, any station tries to lock into signals transmitted at the physical layer by stations belonging to its own group (or groups).
2xe2x80x94The Frame Type field defines the type of the frame: Firstly, it defines: if the frame is a data frame or a control frame. Secondly, it defines the sub-types of frames within each defined type. For data frames the following types are defined: (1) Reserved Data Frames which are frames that are sent using a reservation: exchange-(2) Unreserved data frames which are frames transmitted without going through the full reservation exchange. For Control frames, at least the, following frame types are defined: Request-to-Send (RTS), Clear-to-Send (CTS), End-of-Burst (EOB), End-of-Burst-Confirmed (EOBC), and the Acknowledgement (ACK) frame
3xe2x80x94The Reservation Time field defines the amount of time that a medium is being reserved for. The field is carried in both the RTS and CTS control frames. When used in a data frame it describes the size of the data payload in bytes. This is also known as Block Length (BL). 4. The repetition rate RR defines the best data rate the requesting station should use to send it""s data. Finally the RR* field defines the recommended rate by the target station for sending the data.